Thermally stable polymers with low dielectric constant such as polyimides have been widely used as insulating substrates for use in making flexible printed circuits. By imparting photosensitivity, these polymers can also be used as photoresist in the preparation of laminated printed circuits.
Typically, a laminated integrated circuit is formed by etching a polyimide layer, either to improve its adherence properties or to provide pathways, or the so-called vias, through the polyimide layer to allow for electrical connections to be made between the different layers of metallurgy. An example of such etching process in disclosed in U.S. Pat. No. 4,606,998, in which a metal layer is applied to the polyimide layer in which vias have been formed by anisotropic etching through a photoresist mask. The content of the U.S. Pat No. '998 patent is incorporated by reference.
Generally, the conventional etching method of polyimide type resin layers can be classified into dry etching and wet etching. The dry etching method typically uses O.sub.2 plasma or O.sub.2 sputtering, with which a masking film such as Mo or Cr or other inorganic films are often required. The dry etching method has the advantage that it allows a better etching profile to be obtained. However, the dry etching method also has several drawbacks, such as the complexity of the process and the number of steps required, it the high cost and relatively low etching rate.
The wet etching method, on the other hand, involves a less expensive process and can be employed in a continuous process. The wet etching method typically uses strong acids or alkalines in the etching process. For high performance polymers such as polyimide which cannot be easily etched, the wet etching process often requires the use of highly dangerous chemicals such as hydrazine to degenerate the polymer. One technique for wet etching of fully or substantially fully cured polyimide is to use hydrazine hydrate and a polyamine such ethylene diamine. Wet etching of moderately or slightly cured polyimide (i.e., containing significant amounts of polyamic acids) can be achieved by using a metallic hydroxide. For chemically cured polyimide, in which the packing of the film is not as dense as thermally cured polyimides, wet etching can be effected using relatively highly concentrated hot potassium hydroxide. Because of the high chemical stability and solvent resistance of polyimides, wet etching method typically requires long etching times, in the presence of dangerous and/or explosive chemical solvents, such as hydrazine hydrate and polyamines. Furthermore, in addition to its high toxicity, hydrazine hydrate also exhibits high resin permeability, thus limiting the selection of suitable photoresists that can be used as masking material.
U.S. Pat. No. 4,436,583 discloses a wet etching method of a polyimide type resin film using an etching solution consisting of 20 to 40% by volume of hydrazine hydrate and 60 to 80% polyamine. Hydrazine is known to be a cancer-causing agent, and, as a result, its maximum allowable concentration in air is only less than 0.1 ppm. The use of hydrazine often causes great apprehensions among the workers and is a great environmental concern. Furthermore, because of the high resin permeability of hydrazine, it can invade the photoresist mask and cause distortions of the desired polyimide pattern. The extent of distortion becomes more profound for relatively more fully cured polyimide, or when the polyimide layer has a relatively large thickness. In both instances, longer etching times are required.
U.S. Pat. No. 4,846,929 discloses a wet etching method by which polyimide is etched by contacting the polyimide with an aqueous solution of a metal hydroxide followed by contact with an acid followed by contact again with an aqueous solution of a metal hydroxide. The U.S. Pat. No. '929 patent provides a relatively fast and safe wet etching method for polyimide; however, it can cause severe corrosion problems for the conductive circuits such as copper metals.
U.S. Pat. No. 4,960,491 discloses a method for the wet etching of polymeric materials, in particular polyimide, by reacting the polymer with either an acid or an alkaline etching solution in the presence of a reactive material such as aluminum metal in order to generate nascent hydrogen and improve the etching rate. One of the disadvantages of the method disclosed in the U.S. Pat. No. '491 patent is the generation of a metal-containing liquid pollutant stream. Also this process is difficult to control and the etching rate is greatly affected by the concentration of the acid or alkaline solution.
European Patent Application EP 0373374 discloses a wet etching method by which metallic carbonate is added to a metallic hydroxide etching solution to reduce the extent of undercutting. This method, however, does not improve the etching rate.